Machine tools having automatic control mechanisms for supervision of turning operations are widely used in the industry in conventional lathe machines and the like. Today's industry reflects an ever-increasing criticality for compensation systems for minimizing relatively small, but significant, errors in the position of the workpiece relative the machining tool being applied to that workpiece. Moreover, in multi-spindle lathe applications, it is critical to maintain several machining tools in precise position relative to respective workpieces in order to maintain all parts produced within increasingly close tolerances.
In an effort to enhance productivity while minimizing required core space, multi-spindle lathe machines have replaced single-spindle machines in many high production environments. However, in applications where twin-spindle, two-axis machines are utilized to increase productivity of a particular part, it is imperative that full production be maintained on both spindles in order to justify the significantly higher costs of the two-spindle machines versus the single-spindle machines. The program controlling the movements of the tooling relative the workpieces along the two axes of movement heretofore required that the tools be manipulated simultaneously, necessarily relying on the assumption that identical program motions will produce identical parts. In this regard, it was often required to "qualify" a corresponding pair of tools to assure the identity of their critical dimensions. Unfortunately, however, even identical tools may not perform identically in the machine environment, as a result of idiosyncrasies of the machine itself, differing deflections in use, different wear patterns, and the like.
Heretofore, the industry has attempted to address the problems of these inherent errors by measuring resulting parts and assigning offset errors which can be compensated for by providing adjustable tool blocks, or by undertaking tedious shimming operations of the tools themselves. Often a machinist had no other choice but to average the errors between the two tools, and attempt to adjust the tools and/or tool blocks to compensate. Once these initial errors were reduced sufficiently as a result of such labor-intensive adjustment procedures, it was often necessary to slow the turning process down to preserve tool life and, thereby, delay the tedious process of replacing worn tools as long as possible. Such compromise directly undermined productivity levels, and the process of averaging errors does not generally yield part accuracies which are competitive with the quality of parts made on single-spindle machines, let alone achieving the higher level of accuracy demanded in this industry.
For many years, it has also been recognized that the machining environment can also negatively affect the accuracy of machine tools, such as the effects of temperature on various parts of the machine. Growth of various parts such as spindles, drive screws, and the like can result in significant machine error which limits the accuracy and repeatability of the machine tool. The relatively high temperatures generally concomitant with the machining process causes expansion or "growth" of spindles and the various drives, which causes the components to expand and can result in a change in position of the tool relative the workpiece. This expansion probably occurs over an extended period of time so that the entire error would not be generated and cannot be determined in any single part. The cumulative error which can result from varying tool wear, differences in the tools themselves, and the effects of temperature changes can easily result in an error in the position of the workpiece relative the tool on any particular spindle, which is intolerable in most of today's precision machining operations. In such a case, even if one spindle is producing parts within the tolerance levels required, the substantial premium costs of a two-spindle machine cannot be justified where both spindles are not producing at satisfactory levels.
In the past, those in the industry have attempted to minimize the errors caused by temperature variations by warming the lathe machine up to its "running" temperature before initial offsets are established, and, thereafter running the machine continuously to maintain this stabilized temperature. Another similar approach was to operate the machine continuously to supposedly maintain a constant temperature at all times. It should be noted, however, that stabilization of a lathe may take hours, and assumes that conditions affecting the temperature of the machine will remain substantially constant. In fact, machines may not have a "stabilization" temperature, and oftentimes shutdowns of the machine cannot be avoided. During such a shutdown, the temperature would most likely change. Others have attempted to cool various parts of the machine tool in order to maintain a constant temperature, such as done in a water cooled combustion engine. The cooling systems, however, require additional design features to accommodate the fluid or air necessary for cooling, and would have many of the deficiencies of the other attempts to maintain the machine at a stabilized temperature.
One attempt to provide a compensating system for spindle growth in lathe machine is set forth in U.S. Pat. No. 3,393,588, which issued to F. Broome on July 23, 1968. The Broome reference contemplates a spindle which is movable along a Y-axis of the lathe to adjust the position of a workpiece carried by the spindle relative a cutting tool. The spindle is moved by means of a lead screw, and contemplates the use of a phase detector to feed back the position along the Y-axis from a resolver. A hydraulic motor responds to the error voltage generated by the detector and drives the lead screw to reduce that error. The phase detector thereby puts out an error signal to command the restoration of the spindle to its original position. Broome teaches that expansion of the spindle causes displacement between the chuck and the housing of the spindle, and compensation of that displacement acts to keep the chuck in its original Y position.
The Broome capacitance-gauge arrangement for sensing spindle growth is discussed in U.S. Pat. No. 3,672,246, which issued to H. Prewett, Jr. et al. on June 27, 1972. In particular, Prewett et al. state that the capacitance-type gauge does not always provide a continuously accurate measurement as a result of the relatively hostile environment within a lathe machine. To address the inaccuracies encountered with the Broome compensation device, Prewett et al. describe a device for monitoring a gap between a stationary air jet nozzle and a reference surface on the spindle. In particular, the stationary air jet nozzle detects spindle growth by monitoring the back pressure of the nozzle and converting that input to an electrical signal which is summed with conventional machine control unit carriage position error signal equipment to command an adjustment in the slide offset. It should be noted, however, that the Prewett arrangement is also subject to many of the same inaccuracies of the hostile environment as was Broome, and further requires a relatively cumbersome conglomeration of parts, including an air nozzle, a machined reference surface, air gauges, compressor equipment, valves, air lines, amplifiers, filters, and the like.
Finally, others in the industry have attempted to add on various compensators to the tool block itself, such as mounting one or more of the tool blocks on a separate slide solelY for compensating for small errors. It has been generally found that such add-on compensators do not work well as they are not dependable. Not only do these add-on compensators add greatly to the cost of the machine, but their components are generally smaller, and less able to cope with the harsh environment within a lathe machine. It is this same hostile (i.e., high temperature, messy, laden with machine oil, shavings, and dust) which limits the effectiveness and applications for other error-sensing and feedback mechanisms such as an etched scale and optical reading head often used to monitor growth or contraction of machine parts.
Consequently, heretofore, there has not been available a reliable, low-cost, built-in tool compensating system for lathe machines. Moreover, compensation systems previously available could not effectively provide a multi-spindle machine tool wherein individual process control for each spindle was possible. While multi-spindle machines have been available for quite some time, there has not been presented a compensation system which can consistently maintain high production rates on each spindle in a relatively simple and efficient manner.